A conventional optical element, such as those disclosed in U.S. Pat. No. 9,541,720 B1 and U.S. patent application Ser. No. 15/705,352, includes a light source disposed between a photo detector and an optical fiber. With such arrangement, it is not possible to place the photo detector between the light source and the optical fiber. Moreover, the photo detector of the conventional optical element of U.S. patent application Ser. No. 15/705,352 is located far away from the light source, thereby increasing the overall dimension of an optical instrument including the conventional optical element and the light source.